The present invention relates to system and method for controlling the operation of the boiling water reactor, or more in particular to those for controlling the power distribution thereof.
The boiling water reactor directly supplies the turbine with steam generated within the reactor pressure vessel. A dynamo is connected to the turbine. The reactor pressure vessel contains a core loaded with a multiplicity of fuel assemblies. The cooling water is supplied into the core from the lower side of the core and while being fed upward within the core, cools the fuel rods in the fuel assemblies to be heated to become steam. This steam is supplied to the turbine from the reactor pressure vessel through a steam separator and a dryer at the upper part of the core. After imparting the turning effort to the turbine, the steam is discharged from the turbine and condensed at the condenser. The resulting condensed water is heated at the feed-water heater by the steam bled from the turbine, and is fed through the jet pump in the pressure vessel of the reactor back to the reactor core.
Generally, the nuclear reactor is designed to attain a flat power distribution of the core in order to maintain the soundness of the fuel rods. In the boiling water reactor plant, however, the conditions of bleeding steam from the turbine are different between under the rated power state and under the partial power state. Under the partial power state such as at a start-up time when the flow rate and the power are both low, therefore, the heat balance causes the sub-cooling at the core inlet to be increased as compared with that under the rated power state, thus increasing the power peaking at the lower part of the core. It has been experimentally ascertained that the threshold value for increasing the power by withdrawing the control rods from the reactor core while maintaining the soundness of the fuel rods by preventing them from being damaged is 8 kw/ft in terms of linear heat generation rate. The power increase at a linear heat generation rate of 8 kw/ft or more is effected by increasing the flow rate of the cooling water flowing in the core. At the time of reactor start-up, the power peaking at the lower part of the core increases and therefore the linear heat generation rate may exceed 8 kw/ft. For this reason, in the case where the power is increased by withdrawing the control rods from the core at the start-up time, further withdrawal of the control rods has to be stopped at the time point when the highest point of the power peaking at the lower part of the core reaches 8 kw/ft. Subsequently, as disclosed in U.S. patent application Ser. No. 762,248 filed Jan. 25, 1977 (corresponding to Japanese Patent Application Laid-Open No. 141990/76), the flow rate of the cooling water in the core is increased to attain a desired value of the linear heat generation rate, and thus xenon which is a fission product is accumulated in the fuel rods, so that the flow rate of the cooling water is reduced thereby to reduce the power. In the presence of xenon thus accumulated, the control rods are withdrawn from the reactor core, thus increasing the linear heat generation rate up to 8 kw/ft again. In other words, the sequence of the operation K-L-M-K described in U.S. patent application Ser. No. 762,248 filed Jan. 25, 1977 is repeated. As mentioned above, if the power peaking is large at the lower part of the core, the withdrawal rate of the control rods is reduced and the number of cycles of the operation K-L-M-K necessary for increasing the power of the reactor to the rated value will be increased. This considerably complicates the operation for increasing the power at reactor start-up, thus requiring a long time until the rated power is attained. As a result, the utility of the boiling water reactor is decreased.